A synchronous, clocked communication system with equidistance properties is understood as being a system comprising at least two users interconnected via a data network for the purpose of reciprocally exchanging or, as the case may be, reciprocally transmitting data. Data is thereby exchanged cyclically in equidistant communication cycles predefined by the communication clock employed by the system.
An equidistant deterministic cyclical data exchange in communication systems is based on a clock or, as the case may be, time base shared by all the components participating in the communication. The clock or, as the case may be, time base is transmitted to the other components by an indicated component (clock beater). In the case of isochronous realtime Ethernet the clock or, as the case may be, time base is predefined by a synchronizing master by the transmission of synchronizing telegrams.
Users are, for example, central automation devices, programming, planning or operating devices, peripheral devices such as, for example, input/output modules, drives, actuators, sensors, stored-program controls (SPCs), or other control units, computers, or machines which exchange electronic data with other machines, in particular which process data from other machines. Users are also referred to as network nodes or nodes. Control units are understood below as being regulating or controlling units of any kind, but also, for example, switches and/or switch controllers. Bus systems, for example, including Fieldbus, Profibus, Ethernet, Industrial Ethernet, FireWire, and also PC-internal bus systems (PCI), etc., but in particular also isochronous realtime Ethernet, are employed as data networks.
Data networks make communication between several users possible by networking, which is to say interconnecting individual said users. Communication here denotes the transmission of data between said users. The data being transmitted is thereby sent in the form of data telegrams, which is to say the data is packed into a plurality of packets and sent in this form to the relevant recipient over the data network. The term “data packets” is accordingly also used. The term “trans-mission of data” is here used synonymously with the above-mentioned transmission of data telegrams or data packets.
In distributed automation systems, in the area of, for example, drive engineering, specific data has to be received at specific times by the users for which it is intended and processed by the recipients. The term realtime-critical data or, as the case may be, data traffic is used here because, in contrast to non-realtime-critical, for example internet-based or, as the case may be, intranet-based data communication, failure of the data to arrive at its destination at the proper time will have undesired results for the user. According to IEC 61491, EN61491 SERCOS interface—Brief Technical Description (http://www.sercos.de/deutsch/index deutsch.htm), it is possible to ensure successful realtime-critical data traffic of said type in distributed automation systems.
Today's automation components (for example controls, drives, . . . ) generally have an interface to a cyclically clocked communication system. A flow level of the automation component (fast cycle) (for example position regulation in a control, torque regulation of a drive) is synchronized with the communication cycle. This determines the communication clock. Other, low-performance algorithms (slow cycle) (temperature controls, for example) of the automation component can also only communicate with other components (for example binary switches for ventilators, pumps, . . . ) via this communication clock, although a slower cycle would suffice. The use of only one communication clock for transmitting all the information in the system places stringent demands on the bandwidth of the transmission link.
Very fast and reliable communication systems with predictable response times are essential for process control and monitoring in automated production, and in particular in the case of digital drive systems.
German patent application DE 100 58 524.8 discloses a system and method for transmitting data over switchable data networks, in particular the Ethernet, which permits the mixed operation of realtime-critical and non-realtime-critical communication, in particular internet-based or, as the case may be, intranet-based data communication.
This makes both realtime-critical (RT: Realtime) and non-realtime-critical (NRT: Non-realtime) communication possible in a switchable data network consisting of users and switching units of, for example, a distributed automation system, by means of cyclical operation.
In what is termed a transmission cycle, for all users and switching units of the switchable data network there is in each case at least one time range for transmitting realtime-critical and at least one time range for transmitting non-realtime-critical data, as a consequence of which realtime-critical is separated from non-realtime-critical communication. As users and switching units are always all synchronized with a common time base, the respective time ranges for transmitting data operate at the same time for all users and switching units, which is to say realtime-critical communication takes place temporally independently of non-realtime-critical communication and so is not influenced by it.
Realtime-critical communication is planned in advance. The injection of the data telegrams at the original sender's side and the forwarding of said telegrams by means of the participating switching units takes place on a time basis. Through buffering in the respective switching units, spontaneous, internet-enabled, non-realtime-critical communication arising at any time is relocated to the transmission time range of a transmission cycle provided for non-realtime-critical communication and is only transmitted there.
This patent application presents, by way of example, the embodiment of a basic structure of a transmission cycle which is divided into two time ranges. A transmission cycle is divided into a first time range, which is provided for transmitting realtime-critical data, and a second time range, which is provided for transmitting non-realtime-critical data. The length of the transmission cycle presented symbolizes its temporal duration, which is advantageously between, for example, a few microseconds and a few seconds, depending on the intended purpose.
Although the length of a transmission cycle can be changed, it is specified at least once ahead of the data transmission time by means of, for example, a control computer, and is in each case of equal length for all users and switching units of the switchable data network. The length of a transmission cycle and/or the length of the first time range provided for transmitting realtime-critical data can be changed at any time, for example at pre-planned, fixed times and/or after a planned number of transmission cycles, advantageously before the start of a transmission cycle, by means, for example, of the control computer's switching over to other planned, realtime-critical transmission cycles.
Moreover, the control computer can at any time, depending on the requirements, perform re-planning of realtime communication during ongoing operation of an automation system, as a consequence of which the length of the RT partial cycle can be changed. The absolute length of a transmission cycle remains constant during ongoing operation and is a measure of the temporal portion, or, as the case may be, of the bandwidth of non-realtime-critical communication during a transmission cycle, which is to say of the time available for non-realtime-critical communication.
For a length of realtime-critical communication of 350 μs and a transmission cycle of 500 μs, for example, non-realtime-critical communication accordingly has a bandwidth of 30%, and for 10 ms a bandwidth of 97%. In the first time range provided for transmitting realtime-critical data a certain length of time is reserved prior to transmission of the actual realtime-critical data telegrams for sending data telegrams for organizing the data transmission.
The data telegrams for organizing the data transmission contain, for example, data for synchronizing the users and switching units of the data network and/or data for recognizing the network's topology. The realtime-critical data telegrams are transmitted after these data telegrams have been transmitted. Since realtime communication can be planned in advance by the cyclical operation, the transmission times or, as the case may be, the times for forwarding the realtime-critical data telegrams are known for all realtime-critical data telegrams being transmitted prior to the start of data transmission, which is to say the length of time of the time range for transmitting non-realtime-critical data is automatically determined by the length of time of the time range for transmitting realtime-critical data.
An advantage of this arrangement is that only the necessary transmission time for the realtime-critical data traffic is used in each case and that on completion of said traffic the remaining time is automatically available for non-realtime-critical communication, for example for internet communication which cannot be planned or, as the case may be, for other non-realtime-critical applications.
It is particularly advantageous that the length of time of the time range for transmitting realtime-critical data is in each case determined by the data being transmitted on a connection-specific basis, which is to say the length of time of the two time ranges is determined for each individual data connection by the respectively necessary volume of data of the realtime-critical data being transmitted, as a consequence of which the temporal division of the two time ranges for each individual data connection can be different for each transmission cycle.
Only the necessary transmission time for the realtime-critical data traffic is used in each case and the remaining time of a transmission cycle is automatically available for all users of the switchable data network for non-realtime-critical communication, for example for internet communication which cannot be planned or, as the case may be, for other non-realtime-critical applications.
Realtime communication being appropriately planned in advance such that the realtime-critical data telegrams under consideration arrive at or in the corresponding switching units at or before the forwarding time, said realtime-critical data telegrams can be transmitted or, as the case may be, for warded without a temporal pause, so that optimum use is made of the available length of time thanks to the densely packed transmission, or, as the case may be, forwarding. It is, however, of course also possible to insert pauses in transmission between transmissions of the individual data telegrams.
The basic mode of operation in a switched network is explained—in an exemplary manner by way of representation for any network—below using two users, for example a drive and a control computer, with in each case integrated switching units and another user without a switching unit which are interconnected by means of data connections. The switching units each have local memories connected to the users via internal interfaces.
The users exchange data with the relevant switching units over the interfaces. The local memories are connected within the switching units to the control units via the data connections. The control units receive or, as the case may be, for ward data over the internal data connections from or, as the case may be, to the local memories or via one or more of the external ports. The switching units always have a common synchronous time base as a result of applying the method for synchronizing. If a user has realtime-critical data, said data is fetched at the pre-planned time during the time range for realtime-critical communication via the relevant interface and via the local memory by the relevant control unit and transmitted from there to the next connected switching unit via the external port provided.
If another user transmits non-realtime-critical data, for example for an internet inquiry, at the same time, which is to say during realtime-critical communication, said data is received by the control unit via the external port and forwarded via an internal connection to the local memory and buffered there. Not until during the time range for non-realtime-critical communication is it fetched again from there and for warded to the recipient, which is to say it is relocated into the second time range of the transmission cycle reserved for spontaneous, non-realtime-critical communication, as a consequence of which disruptions to realtime communication are precluded.
For the event of its not being possible to transmit all the buffered, non-realtime-critical data during the time range of a transmission cycle provided for transmitting the non-realtime-critical data, said data will remain buffered in the relevant switching unit's local memory until it can be transmitted during a time range of a later transmission cycle provided for transmitting the non-realtime-critical data, as a consequence of which disruptions to realtime communication are always precluded.
The realtime-critical data telegrams arriving over relevant data connections at the associated switching unit's control unit via the external ports are forwarded directly over the relevant external ports. This is possible because realtime communication has been planned in advance so the transmit and receive time is known for all realtime-critical data telegrams being transmitted, as are all respectively participating switching units, and all times for forwarding, and all recipients of the realtime-critical data telegrams.
It is also ensured thanks to realtime communication having been planned in advance that there will be no data collisions on the data connections. The forwarding times of all realtime-critical data packets from the respectively participating switching units have likewise been planned in advance and hence clearly defined. The arrival of the realtime-critical data telegrams has thus been planned such that the realtime-critical data telegrams under consideration will arrive in the relevant switching unit's control unit no later, or sooner, than the forwarding time. This thereby eliminates the problem of temporal non-clarity apparent particularly in the case of long transmission chains.
It is possible by means of the method described in German patent application DE 100 58 524.8 to set up Ethernet-based communication networks, in particular isochronous Ethernet-based communication networks, whose nodes operate synchronously in the sub-microsecond range, and
which carry out cyclical communication precisely at the planned times (isochronous realtime communication) independently of any other, spontaneous communication
(NRT communication or non-realtime communication) on or, as the case may be, in said network.
All participants in isochronous realtime communication must, however, be based on specific communication hardware in order to
achieve time synchronicity, and
send telegrams precisely at the planned time.
The prior art does not permit the incorporation into isochronous realtime communication, referred to below as IRT communication, of users with an existing Ethernet connection for which said specific hardware has not been provided.